Polyolefin Based Plastic Products in Soil
Over the past 60 years, agricultural output and productivity has significantly increased and plastic materials, mainly polyolefins (e.g. low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polypropylene (PP), and their copolymers and mixtures) made substantial contribution to this development. The main products based on polyolefins are films, drip-irrigation tubing and tapes. For the mulch film alone, it is estimated that about 1 million tons is used worldwide on over 30 million acres of land (P. Halley at al. “Starch” published in 2001 (53), page 362-367). For the USA these numbers in 2004 corresponded to about of 130,000 tons of mulch film usage annually, covering over 185,000 acres of agricultural land (J. P. Warnick at al. “Renewable Agriculture and Food Systems” published in 2006 (21), 216-233). These numbers have continued to grow significantly due to the benefits of mulch films such as increased soil temperature, reduced weed growth, moisture conservation, reduction of certain insect pests, higher crop yields, and more efficient use of soil nutrients.
One major drawback of polyolefins is their resistance to chemical, physical and biological degradation, along with the problem of removal and disposal of agricultural films and other agricultural products after their useful lifetime. If not removed, they tend to accumulate as waste, interfere with root development of the subsequent crop and create serious environmental problems. The cost of removing films from the soil and cleaning them is prohibitively high. This is the main reason why the farmers usually incorporate them into the soil by rototilling, or sometimes burn them in the fields. The problems with disposal of agricultural plastic waste and soil contamination with plastic waste become more and more severe because of increasing usage of plastics.
It is also known that since 2004 well over 1.5 million tons of plastic (primarily polyolefins) mulch film was used in USA. With these practices, it is expected that significant amounts of these plastics are accumulated in soil as waste. It is also known that soils polluted with plastic lose their agricultural value and need to be remediated regularly.
A possible solution to the agricultural plastic waste management would be deployment of biodegradable materials. Biodegradability could be achieved by utilization of soil biodegradable polymers, such as hydrolysable polyesters, e.g. poly(hydroxyalkanoates) (PHA), poly(butylene succinates) and their copolymers. However, despite many years of research and development, these polymers still did not make significant impact in the marketplace due to their inconsistent soil biodegradability and in a majority of cases the necessity of removal and compositing, high cost, life cycle assessment (LCA) and inferior mechanical properties.
A far more promising solution lies in converting polyolefins, the polymers of choice for agricultural film markets, into biodegradable materials that further enriches the agricultural soil with nutrients. This is a quite an ambitious task as polyolefins are known to be bioinert due to their hydrophobicity and high molecular weights.
Transition Metals as Soil Nutrients
Transition metal salts and their mixtures are known to be important plant nutrients: iron, copper, zinc, molybdenum, among others, are needed to support photosynthesis, tolerance to biotic and abiotic stress or nitrogen fixation. However, plants often grow in soils with limited bioavailability (especially of metals in reduced form) and therefore rely on microorganisms for metal uptake. The U.S. Publication. No. 20160311728A1 teaches that coal-derived mineral matter mixed with soil is an effective soil amendment. It increases the silt and clay fractions of the soils and improves soil texture. However, it fails to provide a solution for the soils polluted with non-biodegradable waste.